Coils and what makes up an oscillating electromagnetic wave
The coil Hughes used raised the audio frequency signal on his line to the lower end of the radio band, providing an essential element of our radio definition. How was the frequency raised? Voice, conversations, music, and all other acoustic sounds reside in the the audio frequency band, far below the radio frequency band. Our range of hearing extends to perhaps 20,000 cycles a second, whereas the radio band starts around 100,000 cycles per second, with normal radio frequencies much higher. Let's stop right here to make a distinction between audio or acoustic signals and radio waves.
Sound waves are acoustic waves, with no electrical component. They are simply vibrations in the air, a physical pressure made by the utterance of a speaker or other sound source. Sounds in the audio and radio band both travel in waves but otherwise they are completely dissimilar. Acoustic waves are sounds made manifest by a physical distrubance, electromagnetic or radio waves are the product of radiated electrical energy. Go to this page to read more about acoustic sounds. And this external link from NASA to learn more about radio waves and the entire electromagnetic spectrum:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
When put on a wire a sound occupies the frequency it would normally take up if not on the wire, that is, if a normal conversation is taking place at around 500Hz, then the conversation would naturally set up at 500Hz if put on a wire. That's a simple example, of course, since the telephone system for several reasons limits this baseband or voice band channel on a telephone wire to around 300Hz to 3,000Hz.
As the diagram above show a wire laid flat exhibits only a simple electromagnetic field when current flows. But if you scrunch it together, start running dozens of feet of wire around a core, spacing each loop nearly on top of each other, well, now you've really changed the dynamics of that line. You might have 25 feet or more of wire on a five inch core.
Have you ever seen an A.M. radio antenna in an old style radio? All that wire, wrapped around a ferrite core, is designed to tune frequencies from around 560,000 cycles per second, to about 1,600,000 cycles per second. The length of the wire tries to represent the length of the radio wave itself, although in practice it may be a quarter wavelength in size or less. The closer in size your antenna comes to the size of the wavelength you want to listen to, the better your chances are of receiving it. If you took that same antenna, no core needed, and wired it into a telephone line, you will probably raise the signal on the baseband channel into the low end of the radio band.
Modern radios don't use this principle to produce a high frequency carrier wave, of course, but the point I am making is that an induction coil to produce electromagnetic radio waves was an element which distinguished Hughe's work from more primitive schemes.
So who did complete the first radio telephone call using voice? None other than Alexander Graham Bell, the man who invented the telephone and of course made the first call on a wired telephone to Thomas Watson. Bell was also first with radio, although in a way you probably wouldn't imagine.